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    <script>
        Array.prototype.myMap = function(fn) {
            let arr = []
            for(let i =0;i<this.length;i++) {
                arr.push(fn(this[i],i,this))
            }
            return arr
        }
        const arr = [1,2,3,4]
       const newArr =  arr.myMap(function(item,index,array) {
            return item,index,array
        })
        console.log(newArr);
    </script>
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